The present disclosure relates to a method for producing a fiber preform by deposition of reinforcing fiber bundles on a surface and/or on reinforcing fiber bundles deposited on the surface. The disclosure further relates to the production of a fiber composite component using a fiber preform produced in such a way.
Components made from fiber composites are increasingly used, especially in the aerospace industries, yet also e.g. in the machine building industry or the automotive industry. Fiber composites often offer the advantage of lower weight and/or higher strength over metals. The volume percentage of the reinforcing fibers and especially also the orientation of the reinforcing fibers have a determining effect on the resistance of the components, in particular, the rigidity and strength thereof. Nevertheless, heavy-duty materials and components of this type must still be able to be produced cost effectively in order to be economically attractive.
To produce composite components of this type, so-called fiber preforms are initially produced from reinforcing fibers in an intermediate step. These are textile, semi-finished products in the form of two- or three-dimensional configurations made from reinforcing fibers, wherein the shape can already be nearly the shape of the final component. For embodiments of fiber preforms of this type that consist substantially only of the reinforcing fibers and for which the matrix percentage required for the production of the component is still at least largely absent, a suitable matrix material is incorporated in the fiber preform in additional steps via infusion or injection, or also by application of vacuum. Subsequently, the matrix material is cured as a rule at increased temperatures and pressures to form the finished component. Known methods for infusion or injection of the matrix material are the liquid molding (LM) method, or methods related thereto, such as resin transfer molding (RTM), vacuum assisted resin transfer molding (VARTM), resin film infusion (RFI), liquid resin infusion (LRI), or resin infusion flexible tooling (RIFT). The fiber material used to produce the fiber preforms can also already be pre-impregnated e.g., with small amounts of a plastic material, i.e., a binder material, in order to improve the fixing of the reinforcing fibers in the fiber preform. Pre-impregnated yarns of this type are described for example in WO 2005/095080.
Methods are also known in which composite components are produced from fiber preforms that already have a sufficient content of matrix material for the composite component. In these cases, these fiber preforms can be e.g., compacted directly into the component in a mold using increased pressure and/or increased temperature. Alternatively, it is possible to use a vacuum bag instead of a mold, into which vacuum bag the fiber preform is inserted and, after application of a vacuum and as a rule at increased temperature, is compacted to form the component. The content of matrix material sufficient for the component can, for example, be achieved in that the fiber preform is produced from reinforcing fiber bundles that are produced from prepregs with the corresponding matrix content. Alternatively, during the deposition of e.g., reinforcing fiber bundles to form the fiber preform, additional matrix material can be sprayed on e.g., during the deposition.
To produce fiber preforms from reinforcing fiber bundles, automated processes are often used in which the fiber bundles are deposited by means of controlled deposition heads or also fiber deposition devices on or in corresponding molds, wherein the deposition can also take place by spraying the fiber bundles on or in the molds. As a rule, a continuous yarn of reinforcing fibers is hereby fed to the deposition heads, which yarn is then cut to the desired bundle length in the deposition head or in the fiber deposition device by means of suitable cutting devices. Deposition heads of this type with a device for cutting the fiber strands to length are disclosed, for example, in WO 2011/045172 or U.S. Pat. No. 3,011,257.
Fiber preforms can, for example, be produced in that short-cut reinforcing fibers, together with a binder material, are sprayed and dispersed on an air-permeable screen adapted to the shape of the desired fiber preform, and the fibers are maintained on the screen through the application of vacuum until, after cooling of the binder material, a sufficient stability of the preform is achieved. A method of this type is described for example in WO 98/22644. By means of the method from WO 98/22644, the reinforcing fibers are preferably arranged as short-cut fibers in random, isotropic arrangement and orientation. According to the examples of WO 98/22644, fiber volume fractions only in the range of up to approximately 15 vol. % are achieved, and thus, because of the low fiber volume fractions, only a comparatively low thickness-related strength of the components.
To achieve higher fiber volume fractions in preforms or components produced therefrom, it is advantageous according to the embodiments of WO 2012/072405 to deposit the short-cut fibers in the form of bundles of reinforcing fibers, wherein the fiber bundles preferably have a length in the range from 10 to 50 mm. In addition, it is advantageous, in consideration of the highest possible fiber volume percentages, and thus the highest achievable mechanical characteristics, if the bundles have the lowest possible number of reinforcing fiber filaments, wherein a number of 1000 to 3000 filaments is particularly preferred. In this way, a virtually isotropic material is created with virtually isotropic mechanical characteristics in the directions of extension thereof. At the same time, due to the relatively small bundle dimensions, this material has no or only few regions with increased resin proportion and thus a reduced reinforcing fiber proportion, which regions can lead to weak points in the component. It is relatively easy to see that the use of bundles of reinforcing fibers with low linear density, i.e., with low filament counts, leads to increased costs, in particular due to the use of relatively high-priced source materials as well. On the other hand, although the use of high linear density fiber bundles, i.e., of fiber bundles with a high number of reinforcing fiber filaments, is indeed more cost effective, high fiber volume fractions, as already explained, can be realized only with difficulty, if at all.
There exists therefore a need for an automatable method for producing a fiber preform, by means of which a cost-effective production of fiber preforms is possible while achieving high fiber volume fractions in the fiber preforms or in the composite components produced therefrom.